Passive-adaptive indentor for stress wave cold working

ABSTRACT

A passive-adaptive indentor is provided for cold working a material that includes a material surface and a depth extending in a direction that is perpendicular to a direction of the material surface. The passive-adaptive indentor has a first member that includes a first working end that is configured to create a first residual stress field that extends to a predetermined depth in the material. The passive-adaptive indentor also has a second member that includes a second working end that is configured to create a second residual stress field that extends to a generally fixed second depth of the material regardless of the depth to which the first residual stress field extends. A method of using a passive-adaptive indentor for cold working a material is also presented.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to an apparatus and a method forcold working a material and, more particularly, to an apparatus and amethod for cold working a material by separately applying various forcesto the material in a single application.

[0002] In aerospace structures, materials such as structural membersincluding spars, stringers, ribs and an outer skin, e.g., aluminumsheeting or planar material, may be joined together by the use offasteners. For example, a stringer having an engaging or faying surfacemay be juxtaposed with a corresponding faying surface of a planarmaterial. Thereafter, holes or apertures may be drilled into thematerials and fasteners, such as rivets, inserted into the apertures toform a fastened joint.

[0003] The fastened joints are potentially subject to failure fromfatigue by the formation of fissures or cracks in the materials aftersubstantial use. In airplanes, for example, cyclical pressurization anddepressurization during each flight causes various stress cycles on thefuselage skin. These stress cycles likely contribute to a formation ofcracks, which typically start at the apertures in the material wherefasteners join the airplane skin to the skeleton or rib structure of theairplane. This is because, e.g., varying loads caused by pressurizationchanges that are carried by the skin must be routed around the peripheryof the apertures.

[0004] During assembly of the skin of an airplane, such as along thefuselage and along the wing structure, it is well known to first coldwork the apertures prior to assembly with a fastener in order to createa residual compressive stress about the periphery of the aperture. Theresidual compressive stress functions to counteract a loading thatgenerally includes, in the case of fuselage pressurization changes,tensile strain variations. In this way, the useful life of the fuselageskin is greatly enhanced.

[0005] Current cold working methods require completion of a substantialnumber of steps. For example, a prior art method for cold working amaterial (which may include a joint) is diagrammatically shown at 10 inFIG. 1. In a first drill step 11, a drill 12 is used to create anaperture 14 in a material 16 to be cold worked. In a subsequent inspectstep 18, an inspection device 20 may be employed to inspect the aperture14 to determine if the diameter is within tolerance. Thereafter, in acold work step 22, a mandrel 24 having a diameter that is about 3%larger than that of the aperture 14 is forced through the aperture 14from one side of the material 16. Accordingly, the aperture 14 isthereby expanded which compresses the material surrounding the aperture14 so as to create a residual stress field in the material 16surrounding the aperture 14. In a ream step 28, the aperture 14 may bebored to a proper size for receiving a fastener (not shown) using a ream30. In a counter sink step 32, the aperture 14 is counter sunk with abit 34 to recess a head of a fastener (not shown). Thereafter, in acleaning step 36, a solvent may be used by a mechanic 40 to removelubricating oil from the cold work step, prior to another inspectionstep 42 using the inspection device 20.

[0006] Recently, it has been proposed that the material located, e.g.,at a joint, be compressed or coined, prior to creating an aperture, soas to create a residual stress field which extends within a material tobe fastened. One example of such a two step method is shown in FIG. 2awhere in a coining step 43, a pair of indentors 44 and 46 are urgedunder a force in the direction of arrows 50, 52 into contact with amaterial 48.

[0007] As shown in greater detail in FIG. 2b, the indentors 44 and 46each include a blunt end 54, 56 and a shoulder 58, 60 in fixedrelationship. The blunt ends 54, 56 function to create a residual stressfield, represented by arrows 62, which extends deep within the material48 to a joint 64. The shoulders 58, 60 function to create a largerdiametrical residual stress field, represented by arrows 66, but whichextends generally closely to surfaces 68, 70 of the material 48. The useof shoulders 58, 60 is to overcome the tensile stress created at thepart surface by the blunt ends 54, 56. The depth and configuration ofthe residual stress field is dependent on, e.g., the force applied tothe indentors 44, 46 and a length between the blunt ends 54, 56 and theshoulders 58, 60. Generally, it is desired that the residual stressfield created by the shoulders 58, 60 remain near the surface of thematerial while the stress field created by the blunt ends 54, 56 extendto the joint 64.

[0008] Thereafter and referring again to FIG. 2a, in a drill and countersink step 72, an aperture 74 is created by a drill and a counter sinkbit 76.

[0009] While suitable for its intended purpose, a problem arises in thatthe apparatus and the method illustrated in FIGS. 2a and 2 b requiresemploying a different indentor with a different configuration dependingon the desired depth and configurations of the residual stress fields.This is particularly cumbersome in the manufacture of airplanes as thereare numerous joints to be fastened, most of which, vary in depth fromthe skin.

[0010] Accordingly, there is a need for an improved apparatus and methodfor cold working that does not require the changing of the indentorsdepending on depth of the joint.

SUMMARY OF THE INVENTION

[0011] According to one aspect of the present invention, apassive-adaptive indentor is provided for cold working a material thatincludes a material surface and a depth extending in a direction that isperpendicular to a direction of the material surface. Thepassive-adaptive indentor comprises a first member that includes a firstworking end that is configured to create a first residual stress fieldthat extends to a predetermined depth in the material. The indentor alsocomprises a second member that includes a second working end that isconfigured to create a second residual stress field that extends to agenerally fixed second depth of the material regardless of the depth towhich the first residual stress field extends.

[0012] In another aspect of the invention, a passive-adaptive indentoris provided for cold working a material that includes a material surfaceand a depth extending in a direction that is perpendicular to adirection of the material surface. The passive-adaptive indentorcomprises a first member that has a first working end that is configuredto create a first residual stress field and which extends to apredetermined depth in the material. The first member is subjected to afirst force whereby the first working end strikes the material surfaceat a force sufficient to create the first residual stress field. Asecond member is disposed in slidable relationship with the first memberand comprises a second working end. The second member is configured tocreate a second residual stress field that extends to a generally fixedsecond depth of the material regardless of the depth at which the firstresidual stress field extends. The second member also is subjected to asecond force whereby the second working end strikes the material surfaceat a force sufficient to create the second residual stress field.

[0013] In a further aspect of the invention, a passive-adaptive indentoris provided for cold working a material that includes an aerospacestructure. The aerospace structure includes a planar material having aplanar material faying surface, a support material having a supportmaterial faying surface and a joint located at a contact portion of theplanar material faying surface and the support material faying surface.The planar material includes a planar material surface and a depthextending in a direction that is perpendicular to a direction of theplanar material surface. The passive-adaptive indentor comprises a firstmember that is configured to create a first residual stress field in thematerial that extends to the joint. The first member may be subjected toa first force whereby the first working end strikes the planar materialsurface at a force sufficient to create the first residual stress fieldand the first member may comprise a shaft terminating in a first workingend which comprises an end surface. The indentor may also comprise asecond member that is subjected to a second force whereby the secondworking end strikes the planar material surface at a force sufficient tocreate a second residual stress field that extends to a depth that issubstantially less than that of the first residual stress field. Thesecond member may comprise a tubular structure that terminates in asecond working end that comprises a shoulder surface. The tubularstructure may be dimensioned and configured to be in a co-axialrelationship with the shaft and to be movable along an axial directionof the shaft. The tubular structure may also comprise a collar and aradially extending portion. The indentor may further comprise a housinghaving a cavity defined by an inner surface and an opening and thehousing is in fixed relationship with the shaft which extends within thecavity and through the opening. The tubular structure also extendsthrough the opening. The indentor may still further comprise acompressible spring material which comprises an elastomer that isdisposed between the inner surface of the cavity and a surface of theradially extending portion of the tubular structure.

[0014] In still a further aspect of the invention, a method of coldworking a material using a passive-adaptive indentor to create pluralstress fields in the material is provided. The indentor is capable ofvarying a depth at which a first stress field is to extend while asecond stress field extends to a fixed depth in the material. Thematerial includes a material surface, a depth extending in a directionthat is perpendicular to a direction of the material surface and themethod comprises the steps of: providing a material having multiplelocations to be cold worked; identifying a first location and aparticular depth of interest to which a first stress field is to extendat the first location; identifying a particular force to apply to apassive-adaptive indentor depending upon the particular depth ofinterest; applying the particular force to the first passive-adaptiveindentor to simultaneously create a first residual stress field thatextends to the particular depth of interest and a second residual stressfield that extends to a generally fixed depth of the material regardlessof the depth to which the first residual stress field extends; andmoving the passive-adaptive indentor to another location.

[0015] These and other features, aspects and advantages of the presentinvention will become better understood with reference to the followingdrawings, description and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016]FIG. 1 is a diagram showing a series of steps of a prior artmethod and apparatus for cold working a material;

[0017]FIG. 2a is a diagram showing a series of steps of another priorart method and apparatus for cold working a material;

[0018]FIG. 2b is an enlarged view of a pair of indentors used in themethod and apparatus of FIG. 2a;

[0019]FIG. 3 is a front elevational view of a passive-adaptive indentorin accordance with one embodiment of the present invention;

[0020]FIG. 4 is a cross-sectional view taken along line IV of FIG. 3;

[0021]FIG. 4a is a view similar to FIG. 4 showing a tubular structure ofthe passive-adaptive indentor in a partially retracted position uponcontact with a material to be cold worked;

[0022]FIG. 4b is a view similar to FIG. 4 showing the tubular structureof the passive-adaptive indentor more fully retracted;

[0023]FIG. 4c is a view similar to FIG. 4 showing the tubular structureof the passive-adaptive indentor completely retracted;

[0024]FIG. 5 is a cross-sectional view of a portion of the material tobe cold worked showing a dimple received from the passive-adaptiveindentor of FIG. 3;

[0025]FIG. 6 is a graph showing several instances of dimple depth versusforce applied to the passive-adaptive indentor of FIG. 3 after varyingurethane durability cycles;

[0026]FIG. 7 is a graph showing several instances of true shoulderheight versus force applied to the passive-adaptive indentor of FIG. 3after varying urethane durability cycles;

[0027]FIG. 8 is a cross-sectional view of another embodiment of apassive-adaptive indentor that utilizes a compressible fluid inaccordance with the present invention; and

[0028]FIG. 9 is a cross-sectional view of a further embodiment of apassive-adaptive indentor that utilizes an incompressible fluid andescape orifice in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0029] The following detailed description is of the best currentlycontemplated modes of carrying out the invention. The description is notto be taken in a limiting sense, but is made merely for the purpose ofillustrating the general principles of the invention, since the scope ofthe invention is best defined by the appended claims.

[0030] An improved cold working apparatus and method is provided by thepresent invention that is capable of creating a first residual stressfield that extends to various joint depths depending on the forceapplied to the apparatus while contemporaneously creating a secondresidual stress field. The second residual stress field generallyextends only to a fixed depth that is substantially less than the depthof the first residual stress field. Accordingly, the present inventionprovides a significant advantage by eliminating the required change inindentors because of the variance in joint depth between locations to becold worked.

[0031] A passive-adaptive indentor is shown generally at 100 in FIG. 3in accordance with an embodiment of the present invention. Thepassive-adaptive indentor 100 may comprise a housing 102 and an outputend 104. The housing 102 may be generally cylindrical in outerconfiguration and be composed of any suitably strong and durablematerial capable of withstanding compression forces ranging well above24,000 pounds (24 Kps) such as a hardened steel. The housing 102 mayalso comprise a mounting structure 106 which may be received by acorrespondingly configured receiving structure (not shown) of, e.g., anumerically controlled manufacturing machine (below referred to as an“NC machine”).

[0032] Referring now to FIG. 4 which shows a cross section of thepassive-adaptive indentor 100 where it can be seen that the housing 102may also comprise an interior surface 108 which defines a cavity 110 andan opening 112. The interior surface 108 may comprise a series of stopportions 114, 116 and 118 the function of which is described below.

[0033] The passive-adaptive indentor 100 also may comprise a firstmember or a shaft 120 a second member or tubular structure 122 and aspring material 124. The shaft 120 may be fixed, e.g., via a fastener(not shown) or welding, within the cavity 110 adjacent interior surface108 of the housing 102. However, it will be understood that any othersuitable arrangement whereby the shaft 120 is in fixed relationship withthe housing 102 may be employed.

[0034] The shaft 120 also may be composed of a hardened steel that iscapable of withstanding forces as described above and comprises a firstworking end or end surface 126 and a stop surface 128. The end surface126 is illustrated as generally flat, although, it will be appreciatedthat any suitable configuration may be employed which may create asuitably shaped residual stress field which extends to a desired jointdepth as described below.

[0035] The tubular structure 122 may also be composed of a hardenedsteel and is illustrated as being mounted co-axially with the shaft 120.The tubular structure may be mounted such that it may slide along anouter surface 129 with the shaft 120 and, during use, reciprocates alongthe direction of arrow 130. The tubular structure 120 may comprise acollar 131, an increased radial portion 132, engagement portions 133,134, 136, and 138, and a second working end or shoulder portion 140.

[0036] The collar 131 and increased radial portion 132 together with theinterior surface 108 may define a ring-like space 142 for the receipt ofthe spring material 124, described below. The engagement portions 133,134 and 138 are configured to engage the stop portions 114, 116 and 118,for retaining the tubular structure within the housing 102. Engagementportion 136 may be configured to engage the stop surface 128 of theshaft 120 during reciprocal movement of the tubular structure 122,described below.

[0037] The shoulder portion 140 is illustrated as comprising a surfaceportion 144 that is stepped, although, it will be appreciated that anysuitably configured surface including a generally flat surface may beemployed. It will be appreciated that, during use, the shoulder portion140 causes compression of a larger diameter area of a material,described below, where the head of a fastener (not shown) is located.

[0038] The spring material 124 may be located between the interiorsurface 108 of the housing 102 and the increased radial portion 132 ofthe tubular structure 122. The spring material 124 may be composed of apolymeric or elastomeric substance which is capable of undergoing acompressive strain of roughly 10% of the spring material 124 thickness.Preferably, the spring material 124 may be capable of undergoing acompressive strain that is up to about 15% elastic deformation of itsthickness. A thickness t (FIG. 4) of 1.00 inch is preferred, although,the spring material 124 may range between 0.75 inch and 1.5 inches inthickness. One material that has been found to function suitably is aurethane, No. SG MP950 that is 0.75 inch thick and sold by HarknessIndustries Inc. of (Cheshire), Conn. It will be appreciated that othermaterials, such as a coil spring or a compressible fluid, which providesuitable rebound and elastic deformation may also be employed in thepractice of this invention.

[0039] The operation of this embodiment of the present invention isdescribed with reference to FIGS. 4-4 c. FIG. 4 illustrates a relaxedcondition of the passive-adaptive indentor 100 wherein the tubularstructure 122 is fully extended.

[0040]FIG. 4a illustrates a condition of the passive-adaptive indentor100 after having been forced, in the direction of arrow 145 by asuitable device such as an NC machine, into contact with a material 146to be cold worked, such as a portion of an airplane. In particular, thematerial 146 may comprise a planar material 148 which, e.g., forms the“skin” of an airplane along with a structural support member such as arib or stringer 150 that is to be fastened together with the planarmaterial at a joint 152. It will be appreciated that the shaft 120extends into the planar material 148 a depth that is exaggerated forclarity. As can be seen, the tubular structure 122 has been retractedand the spring material 124 has been compressed somewhat in thedirection of an arrow 154 thereby reducing the force applied to thetubular structure as compared with that applied to the shaft 120. Itwill also be understood that a second passive-adaptive indentor (notshown) similar to the passive-adaptive indentor 100 may be located on anopposing side of the material 146 for providing additional residualstress fields in the material.

[0041] Intermediate and fully retracted conditions of thepassive-adaptive indentor 100 are illustrated in FIGS. 4b and 4 c,wherein the tubular structure 122 moves in the direction of the arrow154 to a position, shown in FIG. 4c wherein the engagement portion 136of the tubular structure 122 is in contact with the stop surface 128 ofthe shaft 120. A dimpling of the material 146 results as is shown inFIG. 5. In particular, a volcano 156 having a height h may be createdalong with a dimple 158 having a stepped configuration and defining atrue shoulder depth ts, a shoulder depth s and a dimple depth d.

[0042]FIG. 6 is a graph depicting the dimple depth versus force appliedto an apparatus in accordance with the present embodiment shown in FIG.4 using a urethane spring material having a thickness of 0.75 inch asdescribed above. As can be seen from this graph, a generally linearrelationship exists between dimple depth and force applied from about6,000 pounds of force to about 13,000 pounds of force. FIG. 7 is a graphshowing the true shoulder depth versus force applied and illustratingthat, in the range of between 6,000 pounds of force and 13,000 pounds offorce very little shoulder depth occurs as compared with the dimpledepth shown in FIG. 6 for the same range.

[0043] Referring now to FIG. 8, another embodiment of a passive-adaptiveindentor in accordance with the present invention is shown generally at200. Similar to the passive-adaptive indentor 100, the passive-adaptiveindentor 200 may comprise a housing 202, a first member or shaft 204 anda second member or tubular structure 206. However, in addition to theforegoing, the passive-adaptive indentor 200 may further comprise apiston 208 that may be connected to the tubular structure 206 via, e.g.,fasteners 209.

[0044] The housing 202 may be composed of a hardened steel and comprisesan inner surface 210 that defines a cavity 212, a closure member 214that comprises an opening 216 and an access bore 218. Rather than asolid spring material as described above a compressible fluid 219, suchas air, may be filled through the access bore 218 and into the cavity212 for providing a resilient cushion for the tubular structure 206. Afill screw 220 may be provided for closing the access bore 218. A returnspring (not shown) such as a coil spring may also be disposed within thecavity 212 for urging the piston 208 adjacent the closure member 214.The closure member 214 may be mounted to the housing 202 via anysuitable means including, e.g., fasteners 222.

[0045] The shaft 204 may be affixed to the housing via a fastener 224and may function similar to the shaft 120 described above with respectto the passive-adaptive indentor 100. Therefore, reference may be had tothe above description for further details. The tubular structure 206 maybe mounted co-axially and in sliding engagement with the shaft 204 andmay function similar to the tubular structure 122. Excepting that inthis embodiment, the tubular structure 206 is connected to piston 208that comprises a collar 226 and a radially extending portion 228. Thepiston 208 may also slide along the shaft 204 and comprises seals 230for sealing the compressible fluid within the cavity 212.

[0046] The operation of the passive-adaptive indentor 200 is similar tothat described above with respect to the passive-adaptive indentor 100,although, in the present embodiment, the compressible fluid 219 maycushion movement of the tubular structure 206 thereby providing areduced force by the tubular structure 206 as compared with that of theshaft 204.

[0047] A further embodiment of an passive-adaptive indentor inaccordance with the present invention is illustrated generally at 300 inFIG. 9. The passive-adaptive indentor 300 may be similar to thepassive-adaptive indentor 200 described above, although, thepassive-adaptive indentor 300 may further comprise a fluid reservoir302, an orifice 304 and a return spring 306. The fluid reservoir 302 mayfunction to provide a reservoir for the flow of an incompressible fluid308, such as a hydraulic fluid, out of a cavity 310 during compressionthereof. The orifice 304 may be provided to regulate that flow andthereby provide a particular back-pressure of incompressible fluid 308within the cavity 310. The spring material 306 may be composed of anelastomer or other suitable material as described above in connectionwith the spring material 124.

[0048] In operation, the passive-adaptive indentor 300 may functionsimilar to that described above with respect to the passive-adaptiveindentor 200 excepting that the back pressure of incompressible fluid308 and spring material 306 combine to reduce the net force applied tothe tubular structure 312 as compared to that of the shaft 314.

[0049] In still a further embodiment of the present invention, a methodof cold working a material using a passive-adaptive indentor such asdescribed above to create plural stress fields in the material isprovided. The method may comprise the steps of providing a materialhaving multiple locations to be cold worked. Thereafter, identifying afirst location and a particular depth of interest to which a firststress field is to extend at the first location. Next, identifying aparticular force to apply to a passive-adaptive indentor depending uponthe particular depth of interest. Thereafter, applying the particularforce to the first passive-adaptive indentor to simultaneously create afirst residual stress field that extends to the particular depth ofinterest and a second residual stress field that extends to a generallyfixed depth of the material regardless of the depth to which the firstresidual stress field extends. Finally, moving the passive-adaptiveindentor to another location.

[0050] The method may also comprise the step of locating a secondpassive adaptive indentor on an opposing side of the material from thefirst passive adaptive indentor. Thereafter, applying the particularforce to the second passive-adaptive indentor to simultaneously create athird residual stress field that extends to the particular depth ofinterest and a fourth residual stress field that extends to a generallyfixed depth of the material regardless of the depth to which the thirdresidual stress field extends.

[0051] It should be understood, of course, that the foregoing relates topreferred embodiments of the invention and that modifications may bemade without departing from the spirit and scope of the invention as setforth in the following claims.

We claim:
 1. A passive-adaptive indentor for cold working a material,the material including a material surface and a depth extending in adirection that is perpendicular to a direction of the material surface,comprising: a first member comprising a first working end and beingconfigured to create a first residual stress field that extends to apredetermined depth in the material; and a second member comprising asecond working end and being configured to create a second residualstress field that extends to a generally fixed second depth of thematerial regardless of the depth to which the first residual stressfield extends.
 2. The passive-adaptive indentor of claim 1 wherein: thematerial comprises an aerospace structure having at least one planarmaterial including a planar material surface and a planar materialfaying surface, a support member having a support member faying surfaceand a joint located at a contact portion of the material faying surfaceand the support member faying surface; and the first residual stressfield that is created extends at least to a depth of the joint and thesecond residual stress field that is created extends to a depth that issubstantially less than that of the first residual stress field andsubstantially adjacent the planar material surface.
 3. Thepassive-adaptive indentor of claim 1 wherein the second member isdisposed in slidable relationship with the first member.
 4. Thepassive-adaptive indentor of claim 1 wherein: the first member issubjected to a first force whereby the first working end strikes thematerial surface at a force sufficient to create the first residualstress field; and the second member is subjected to a second forcewhereby the second working end strikes the material surface at a forcesufficient to create the second residual stress field.
 5. Thepassive-adaptive indentor of claim 4 wherein: the first member comprisesa shaft terminating in the first working end which terminates in an endsurface; and the second member comprises a tubular structure terminatingin the second working end, the second working end comprising a shouldersurface and wherein the tubular structure is dimensioned and configuredto be in a co-axial relationship with the shaft and be movable along anaxial direction of the shaft.
 6. The passive-adaptive indentor of claim5 further comprising: a housing comprising a cavity defined by an innersurface, the housing being in fixed relationship with the shaft; and aspring material disposed within the cavity, the spring material beinginterposed between the inner surface of the housing and the tubularstructure.
 7. The passive-adaptive indentor of claim 6 wherein thehousing comprises a support mount and an opening and wherein the shaftis fixed to the inner surface of the cavity and the shaft and thetubular structure extend through the opening.
 8. The passive-adaptiveindentor of claim 6 wherein the tubular structure comprises a collar anda radially extending portion and wherein the spring material is disposedbetween the inner surface of the cavity and a surface of the radiallyextending portion.
 9. The passive-adaptive indentor of claim 8 whereinthe spring material comprises a polymer.
 10. The passive-adaptiveindentor of claim 8 wherein the spring material is capable of undergoingelastic deformation that is within the range of between about 0% andabout 15%.
 11. The passive-adaptive indentor of claim 8 wherein thespring material is capable of undergoing about 15% elastic deformation.12. The passive-adaptive indentor of claim 8 wherein the spring materialcomprises an elastomer.
 13. The passive-adaptive indentor of claim 8wherein the spring material comprises a compressible fluid.
 14. Thepassive-adaptive indentor of claim 8 wherein the housing comprises afluid reservoir and a fill bore which comprises a reduced diametricalportion.
 15. The passive-adaptive indentor of claim 14 furthercomprising an incompressible fluid.
 16. The passive-adaptive indentor ofclaim 6, further comprising: a piston being movably disposed within thecavity and being interconnected for movement with the structure andwherein the piston is disposed between the spring material and thetubular structure.
 17. The passive-adaptive indentor of claim 16 whereinthe piston comprises a collar having a central bore and a radiallyextending portion and wherein the spring material is disposed betweenthe inner surface of the cavity and a surface of the radially extendingportion.
 18. A passive-adaptive indentor for cold working a material,the material including a material surface and a depth extending in adirection that is perpendicular to a direction of the material surface,comprising: a first member comprising a first working end and beingconfigured to create a first residual stress field that extends to apredetermined depth in the material, the first member being subjected toa first force whereby the first working end strikes the material surfaceat a force sufficient to create the first residual stress field; and asecond member being disposed in slidable relationship with the firstmember and comprising a second working end, the second member beingconfigured to create a second residual stress field that extends to agenerally fixed second depth of the material regardless of the depth atwhich the first residual stress field extends, the second member beingsubjected to a second force whereby the second working end strikes thematerial surface at a force sufficient to create the second residualstress field.
 19. The passive-adaptive indentor of claim 18 wherein: thefirst member comprises a shaft terminating in the first working endwhich comprises an end surface; and the second member comprises atubular structure terminating in the second working end, the secondworking end comprising a shoulder surface and wherein the tubularstructure is dimensioned and configured to be in a co-axial relationshipwith the shaft and be movable along an axial direction of the shaft. 20.The passive-adaptive indentor of claim 19 wherein the tubular structurecomprises a collar and a radially extending portion and furthercomprising: a housing having a cavity defined by an inner surface and anopening, the housing being in fixed relationship with the shaft whichextends within the cavity and through the opening, the tubular structurealso extending through the opening; and a compressible spring materialdisposed within the cavity, the compressible spring material beinginterposed between the inner surface of the cavity and a surface of theradially extending portion of the tubular structure.
 21. Thepassive-adaptive indentor of claim 20 wherein the compressible springmaterial comprises an elastomer disposed within the cavity of thehousing.
 22. The passive-adaptive indentor of claim 20 wherein thecompressible spring material is capable of undergoing elasticdeformation that is within the range of between about 0% and about 15%.23. The passive-adaptive indentor of claim 20 wherein the compressiblespring material is capable of undergoing about 15% elastic deformation.24. The passive-adaptive indentor of claim 20 wherein the compressiblespring material comprises an elastomer.
 25. The passive-adaptiveindentor of claim 20 wherein the piston comprises a collar having acentral bore and a radially extending portion and wherein thecompressible spring material is disposed between an inner surface of thecavity and a surface of the radially extending portion.
 26. Apassive-adaptive indentor for cold working a material, the materialcomprising an aerospace structure having a planar material including aplanar material faying surface, a support member having a support memberfaying surface and a joint located at a contact portion of the planarmaterial faying surface and the support member faying surface, theplanar material including a planar material surface and a depthextending in a direction that is perpendicular to a direction of theplanar material surface, comprising: a first member being configured tocreate a first residual stress field in the material that extends to thejoint, the first member being subjected to a first force whereby thefirst working end strikes the planar material surface at a forcesufficient to create the first residual stress field and the firstmember comprising a shaft terminating in a first working end whichcomprises an end surface; a second member being subjected to a secondforce whereby the second working end strikes the planar material surfaceat a force sufficient to create a second residual stress field thatextends to a depth that is substantially less than that of the firstresidual stress field and wherein the second member comprises a tubularstructure which terminates in a second working end that comprises ashoulder surface and wherein the tubular structure is dimensioned andconfigured to be in a co-axial relationship with the shaft and bemovable along an axial direction of the shaft, the tubular structurealso comprising a collar and a radially extending portion; a housinghaving a cavity defined by an inner surface and an opening, the housingbeing in fixed relationship with the shaft which extends within thecavity and through the opening, the tubular structure also extendingthrough the opening; and a compressible spring material comprising anelastomer being disposed between the inner surface of the cavity and asurface of the radially extending portion of the tubular structure. 27.A method of cold working a material using a passive-adaptive indentor tocreate plural stress fields in the material and that is capable ofvarying a depth at which a first stress field is to extend while asecond stress field extends to a fixed depth in the material, thematerial including a material surface, a depth extending in a directionthat is perpendicular to a direction of the material surface, the methodcomprising the steps of: providing a material having multiple locationsto be cold worked; identifying a first location and a particular depthof interest to which a first stress field is to extend at the firstlocation; identifying a particular force to apply to a passive-adaptiveindentor depending upon the particular depth of interest; applying theparticular force to the first passive-adaptive indentor tosimultaneously create a first residual stress field that extends to theparticular depth of interest and a second residual stress field thatextends to a generally fixed depth of the material regardless of thedepth to which the first residual stress field extends; and moving thepassive-adaptive indentor to another location.
 28. The method of claim27 wherein: the material comprises an aerospace structure having aplanar material including a planar material faying surface, a supportmaterial having a support material faying surface and a joint located atthe contact portion of the planar material faying surface and thesupport material faying surface; and the first residual stress fieldcreated extends to the depth of the joint and the second residual stressfield is created at about the material surface.
 29. The method of claim27 further comprising the steps of: locating a second passive adaptiveindentor on an opposing side of the material from the first passiveadaptive indentor; and applying the particular force to the secondpassive-adaptive indentor to simultaneously create a third residualstress field that extends to the particular depth of interest and afourth residual stress field that extends to a generally fixed depth ofthe material regardless of the depth to which the third residual stressfield extends.